The present invention relates to a circuit configuration for controlling the pressure decrease in an anti-locking control system, comprising a master brake cylinder connected to the brake wheels through multi-way valves open in the initial position and reswitchable to blocking, and a brake pressure modulator coupled upstream of the master brake cylinder. The circuit configuration is controllable in accordance with a multiplex process, with the multi-way valves leading to the wheel brakes being reswitchable upon commencement of the anti-locking control to the blocking position and with the pressure in the master brake cylinder being reducible.
In known systems of this type, the pressure modulator is provided as an assembly operated by hydraulic auxiliary pressure or vacuum, which assembly serves both for brake force boosting and for generating an auxiliary force counter-acting the pedal force. In the event of slip control, the assembly reduces the pressure in the working chambers of the master brake cylinder (DE-OS 33 17 629; DE OS 36 41 105; and, German Patent Application P 38 22 260.4). Brake systems of this type are operated in the known multiplex process and therefore need only a relatively small number of brake pressure control valves. One multiway valve open in the initial position and reswitchable to blocking is provided in the pressure fluid conduit leading from the master brake cylinder to one or to two wheel brakes, respectively. Upon commencement of the anti-locking control, first, all valves leading to the wheel brakes are reswitched; subsequently, the brake pressure in the master cylinder is decreased and only the wheel valve which leads to the instable wheel is reopened. Consequently, the brake pressure in the wheel brake of that wheel will decrease while, in the rest state of the wheel brakes, it will remain constant. In the same manner, the brake pressure in the other wheel brakes will be successfully modulated, that is, decreased, kept constant or further increased.
During blocking of the inlet valves in this initial phase of the anti-locking control, which coacts with the development of the auxiliary force counter-acting the pedal force and, in some cases, with a blocking or de-coupling of the pedal (Patent Application P 38 22 260.4), at least in unfavorable conditions, the pressure in the master brake cylinder further rises or excessively increases. This is disadvantageous because the pressure decrease in the wheel tending to lock, which is required in that phase, thereby is delayed thus affecting the control and the control quality, respectively.
It is, therefore, an object of the present invention to overcome this disadvantage while avoiding the need for substantial additional efforts and avoiding still other disadvantages. It has been found that this problem is solved by a circuit configuration of the type referred to in the foregoing, a particular feature of which is that, in the first control phase, if the first wheel tending to lock is a front wheel, the reswitch of the wheel valves leading to the rear wheel brakes is delayed by a predetermined period of time.